Taking the flexible approach to power conversion circuit design
For a design engineer, everything can look like an application. And as any engineer will confirm, your design choices are always application-dependent. Sometimes those choices are easy because some components are more general-purpose than others. The rest of the time, the choice is harder, even when the application is very clear about what it wants.
Power is a good example. Many applications now require more power, delivered through multiple power rails. These requirements are typically met using auxiliary power supplies running either off-line or from another voltage source.
Industrial applications undergoing the shift to electrification will likely have significant demand for auxiliary power. In the automotive area, vehicles now use auxiliary power for the various electric motors used to pump fluids, wind windows, or move seats. Robotics is an application defined by electric motors, all of which need power in various forms and sometimes a lot of it.
And let’s not forget the process of generating power. Renewable energy is all about managing, converting, storing, and transferring energy in the form of AC and DC voltages and currents. It can involve multiple stages of conversion and each stage has its own power requirements.
When we think about designing an auxiliary power supply, we quickly reach the point where decisions need to be made. With any single project presenting multiple power requirements, many of those choices will need to be made several times.
Although power components are often designed with very specific performance metrics in mind, sometimes there are solutions presented that are more flexible than others. This gives design engineers the freedom to make some of their design choices a little later in the project, or even change the decision based on evolving requirements. It also means the same, flexible part can be used in multiple places.
One of the bigger design choices power design engineers need to make today is what kind of transistor to use. The options have grown in recent years, with the introduction of silicon carbide MOSFETs and Gallium Nitride high electron mobility transistors (HEMT). They join the familiar silicon and super-junction MOSFETs, and the IGBT.
Often, and with good reason, engineers focus on the type of transistor. There is a lot of momentum behind using SiC in power applications, for example, because they offer significant benefits over silicon MOSFETs. The choice may also dictate other design decisions, due to the requirements wide bandgap materials have over regular silicon.
The power supply topology selected also plays a role in directing design decisions. For auxiliary power suppliers, the isolated flyback topology is a popular choice. This flexible design supports buck (step-down) and boost (step-up) configurations. The power supply design can be relatively simple, or more advanced by incorporating quasi-resonant switching, which may help decrease electrical noise in sensitive applications. The flyback converter can also be configured as either insulated or non-insulated, depending on the application’s requirements.
These possible variations might suggest that the control solution used must be dedicated to one configuration. That might be the case for some solutions, but a new PWM controller introduced recently by ROHM Semiconductor offers more flexibility than some others on the market. The BD28C5xH/LFJ-LB family of controllers for PWM-based DC/DC or AC/DC converters may be aimed at auxiliary power suppliers, but it brings a high degree of design freedom.
Variants of the controller feature different undervoltage lock-out levels to support safe gate driving of a wide variety of power semiconductors, including IGBT, silicon, and SiC MOSFETs. It also supports current detection, so can be used in current mode control configurations.
The PWM switching signal frequency and ratio are set using an external resistor/capacitor network, connected to the part’s own voltage reference pin. The network feeds into the RTCT pin, in the form of a triangular wave. The maximum PWM frequency is 100 kHz, with a maximum mark/space ratio of 50%.
An internal error amplifier feeds into the PWM comparator, and the signal is also routed to an output pin. This can be used to implement a soft start function using a small number of external components. Driving the pin low will disable the output.
The BD28xFJ-LB from ROHM
The current sense pin feeds into an integrated current-to-voltage converter and used to implement over-current protection. By converting the RTCT triangular waveform to a digital on/off signal using an external transistor and potential divider network, the signal can be used on the current sense pin to operate in voltage mode.
In all, the BD28C5xH/LFJ-LB family offers a lot of functionality and flexibility, in a small 8-pin SOP-J8 surface-mount package. Contact your Avnet Silica representative to find out more, request samples or explore power supply solutions based on this PWM controller and other solutions from ROHM.
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